Lead acetate-paper test

Lead Acetate Test Paper Usually about 6 x 80 mm in size. Use Lead Acetate TS, and dry the paper at 100°, avoiding contact with metal. 

In the lead acetate test (ASTM D2420), the vaporized gas is passed over moist lead acetate paper under controlled conditions. Hydrogen sulfide reacts with lead... 

Carbon dioxide (ASTM D-1137, ASTM D-1945, ASTM D-4984) in excess of 3% is normally removed for reasons of corrosion prevention (ASTM D-1838). Hydrogen sulfide (ASTM D-2420, ASTM D-2385, ASTM D-2725, ASTM D-4084, ASTM D-4810, IP 103, IP 272) is also removed, and the odor of the gases must not be objectionable (ASTM D-6273) so mercaptan content (ASTM D-1988, ASTM D-2385, IP 272) is important. A simple lead acetate test (ASTM D-2420, ASTM D-4084) is available for detecting the presence of hydrogen sulfide and is an additional safeguard that hydrogen sulfide not be present (ASTM D-1835). Methyl mercaptan, if present, produces a transitory yellow stain on the lead acetate paper that fades completely in less than 5 min. Other sulfur compounds (ASTM D-5504, ASTM D-6228) present in liquefied petroleum gas do not interfere. 

In the lead acetate test (ASTM D-2420), the vaporized gas is passed over moist lead acetate paper under controlled conditions. Hydrogen sulfide reacts with lead acetate to form lead sulfide, resulting in a stain on the paper varying in color from yellow to black depending on the amount of hydrogen sulfide present. Other pollutants can be determined by gas chromatography (ASTM D-5504, ASTM D-6228, IP 318). 

These methods can be classified as absence/presence tests , in terms of microbiological tests. The tests of this kind that are most frequently used in the field are an acid test and a lead acetate test . In an acid test, a few droplets of diluted hydrochloric acid are added on the corrosion products. If there is a smell of rotten egg - characteristic of hydrogen sulphide gas - then the corrosion products do contain sulphides. The lead acetate test is basically using moist lead acetate [Pb(C2H302)2-3H20] papers. In contact with sulphides, these papers turn black. 

Upon fusion with caustic alkah (for experimental details, see Section IV,33,2) and acidification of the aqueous extract, hydrogen sulphide is evolved (detected by lead acetate paper). This test is given by aU organic compounds of divalent sulphur (RSH, R SR" and R SSR"). 

This solution may also be employed in the test for bromine. If iodine has been found, add small amounts of sodium nitrite solution, warm shghtly and shake with fresh 1 ml. portions of carbon tetrachloride until the last extract is colourless boil the acid solution until no more nitrous fumes are evolved and cool. If iodine is absent, use 1 ml. of the fusion solution which has been strongly acidified with glacial acetic acid. Add a small amount of lead dioxide, place a strip of fluorescein paper across the mouth of the tube, and warm the solution. If bromine is present, it will colour the test paper rose-pink (eosin). 

When an organic compound is heated with a mixture of zinc powder and sodium carbonate, the nitrogen and halogens are converted into sodium cyanide and sodium hahdes respectively, and the sulphur into zinc sulphide (insoluble in water). The sodium cyanide and sodium hahdes are extracted with water and detected as in Lassaigne s method, whilst the zinc sulphide in the residue is decomposed with dilute acid and the hydrogen sulphide is identified with sodium plumbite or lead acetate paper. The test for nitrogen is thus not affected by the presence of sulphur this constitutes an advantage of the method. 

Materials Required Arsenic limit test apparatus HgCl2—paper smooth white filter paper (having thickness in mm of 400 paper = weight in g per Sq. M.), soaked in a saturated solution of HgCl2, pressed to get rid of excess of soln. and dried at about 60°C in the dark lead acetate solution 10.0% w/v soln. of PbAc2 in C02 free water KI (AsT), 1.0 g Zn (AsT) 10.0 g Dilute Arsenic solution (AST) Standard stains, Test Solutions—are prepared according to the Indian Pharmacopoeia 1996. 

Corrosive sulfnr componnds can be detected by their effect on copper and the form in which the general copper strip corrosion test (ASTM D1838) for petrolenm prodncts is applied to liqnefied petrolenm gas. Hydrogen sulfide can be detected by its action on moist lead acetate paper, and a procedure is also used as a measure of sulfur compounds. The method follows the principle of the standard Doctor test. 

Reaction with hydrogen sulfide forms black precipitate of lead sulfide, PbS. A paper soaked with lead acetate solution turns black on exposure to H2S, a test often used to detect sulfide. 

Elemental composition Na 58.93%, S 41.07%. An aqueous solution is analyzed to determine sodium content. Also, an aqueous solution may be analyzed for sulfide by methylene blue colorimetric test or by iodometric titration (APHA, AWWA, and WEF. 1999. Standard methods for the Examination of Water and Wastewater, 20 ed. Washington, DC American Pubhc Health Association). The methylene blue test is based on reacting sulfide, ferric chloride and dimethyl-p-phenylenediamine to produce methylene blue. Also, sulfide can be measured by using a sdver-sdver sulfide electrode. Quahtatively, sulfide may be identified from the hberation of H2S on treatment with acid. The H2S turns the color of paper soaked with lead acetate black (See Hydrogen Sulfide). 

Note According to one of the Hercules Powder Co laboratory manuals, lead acetate is used in the prepn of test paper for detection of hydrogen sulfide. Strips of filter paper are dipped in 10% Pb acetate soln and then dried in atm free of HaS. The moist test paper, in the presence of even traces of HaS, gives a black color... 

Hydrogen Sulphide as an Acid. — In the anhydrous condition, whether as gas or liquid, hydrogen sulphide has no acidic properties. When moist or in aqueous solution, however, it behaves as a feeble acid, whence the occasional name hydrosulphuric acid. For the characteristic test with lead acetate paper or for the reddening of litmus paper, the presence of at least a little moisture is essential.3... 

Hydrogen Sulphide. — Heat 1 gm. of animal charcoal with 40 cc. of water and 10 cc. of hydrochloric acid, and test the escaping vapors with moistened lead acetate paper. The latter should not acquire a brown color. 

Detection of Sulphur Dioxide.—This test is made on sugars from juices decolorised by means of sulphurous acid. To a solution of 10-15 grams of the sugar in about 25 c.c. of water in a flask are added a scrap of pure zinc (or 0-3-0-4 gram of magnesium wire) and 5 c.c. of pure hydrochloric acid. In presence of sulphur dioxide, hydrogen sulphide is evolved and may be detected by the odour or by means of a strip of lead acetate paper. 

Test for sulphide dyestuffs. The sample is covered in a test-tube or conical flask with the acid stannous chloride solution, the mouth of the vessel being closed with a piece of filter-paper steeped in lead acetate solution and the liquid heated slowly to boiling. The appearance of a blackish-brown stain on the filter-paper indicates a sulphur dyestuff. The boiling should not be too protracted, since otherwise the hydrochloric acid vapours may cause the brown lead sulphide to disappear. 

Test Papers. See under Methyl Violet Tests in Vol 8, M118-Lto M120-L, and Lead Acetate in Vol 1, A28-R... 

Hydrogen sulfide gas evolved, detected by holding lead acetate paper to mouth of test tube Evolution of sulfur dioxide gas Red compound of unknown composition... 

Zinc and sulphuric acid hydrogen sulphide gas is evolved, which may be detected by holding lead acetate paper to the mouth of the test-tube (cf. Section IV.6, reaction 1) ... 

Place a drop of the test solution together with a few milligrams of lead dioxide and acetic acid in the apparatus of Fig. 11.56 and close the tube with the funnel stopper carrying a piece of filter paper which has been impregnated with the reagent and dried. Warm the apparatus gently. A circular red spot is formed on the yellow test paper. 

The test is usually carried out by adding the reagent to the solution acidified with dilute hydrochloric acid carbonates, sulphites, and phosphates are not precipitated under these conditions. Concentrated hydrochloric acid or concentrated nitric acid should not be used, as a precipitate of barium chloride or of barium nitrate may form these dissolve, however, upon dilution with water. The barium sulphate precipitate may be filtered from the hot solution and fused on charcoal with sodium carbonate, when sodium sulphide will be formed. The latter may be extracted with water, and the extract filtered into a freshly prepared solution of sodium nitroprusside, when a transient, purple colouration is obtained (see under Sulphides, Section IV.6, reaction 5). An alternative method is to add a few drops of very dilute hydrochloric acid to the fused mass, and to cover the latter with lead acetate paper a black stain of lead sulphide is produced on the paper. The so-called Hepar reaction, which is less sensitive than the above two tests, consists of placing the fusion product on a silver coin and moistening with a little water a brownish-black stain of silver sulphide results. 

---